1. Field of the Invention
The present invention relates to a sound recording apparatus, and more particularly, to a standard (normal) sound signal recording apparatus for use in video tape recorders (VTRs).
2. Description of the Prior Art
In VTRs, a standard sound signal is equalized before being recorded onto a sound track of a magnetic tape. The equalization is performed in accordance with a recording speed mode such as an extended play (EP) mode or a standard play (SP) mode.
Referring to FIG. 1, there is shown a conventional circuit where the equalization is performed in accordance with the recording speed mode. Numeral 1 is an operational amplifier. To a non-inverting input terminal (+) thereof, a sound signal is supplied. To an inverting input terminal (-) thereof, a negative feedback is applied from an output terminal thereof via resistors R3 and R4. A first series circuit 2 consisting of an inductance coil L, a capacitor C1 and a resistor R1 which are discrete parts is connected to the inverting input terminal (-) of the operational amplifier 1 and to ground. A second series circuit 4 consisting of a capacitor C2, a resistor R2 and a switch 3 is connected to the inverting input terminal (-) via a node (a) between the inductance coil L and the capacitor C1, and to ground.
In the SP mode, the switch 3 is OFF, so that the capacitor C2 and the resistor R2 constituting the second series circuit 4 are disabled and only the first series circuit 2 is activated. Under this condition, the resonant frequency takes a relatively high value f1 in middle and high bands as shown in FIG. 2. The gain (voltage gain) of the operational amplifier 1 is boosted in such manner that the frequency f1 is the center frequency.
That is, the resonant frequency f1 is ##EQU1## A gain Gv1 in the vicinity of the frequency f1 (i.e. in the boosted band) is ##EQU2##
In the EP mode, the switch 3 is ON, so that the second series circuit 4 is activated as well as the first series circuit 2. Under this condition, the resonant frequency takes a relatively low value f2 in the middle and high bands. The gain of the operational amplifier 1 is boosted in such manner that the frequency f2 is the center frequency.
That is, the resonant frequency f2 is ##EQU3## A gain Gv2 in the vicinity of the frequency f2 is ##EQU4##
Thus, in each recording speed mode, the sound signal is equalized in such manner that the predetermined band is boosted. Then, the equalized sound signal is directed to a succeeding circuit (not shown), where it is recorded onto a sound track of a magnetic tape through a magnetic head.
In FIG. 2, the portion of the band which is still higher than the middle and high bands is not boosted. This is because no standard sound signals for VTRs are present in that portion.
In the conventional circuit where the discrete inductance coil L is used on the negative feedback side of the operational amplifier 1, it is highly probable that electromagnetic induction with an external environment (e.g. a television receiver) occurs through the inductance coil L. Further, the cost of the inductance coil L which is comparatively high compared to those of capacitors and resistors increases the cost of the entire apparatus. Moreover, an additional work is required to connect the coil L.